Yeah, Excitebike64 has one of the best course editor, but only stadium circuits through.
As for MK Wii that is very easy to do.
If Nintendo allows users to move the set pieces of SNES and GBA courses like in the Excite bike editor, then we have everything we need to start with.
I don't expect it will be easy for the already 3d courses of MK 64 and its cohorts, since there are too much unique set pieces to make it happen.
We could even build SNES and GBA tracks from F-zero and other race games.